[bug#27935] gnu: Add r-shiny.

[Ricardo Wurmus]

Hi Guix,

this patch series adds r-shiny and packages for the minified JavaScript
files that it bundles.

For each of the minified JavaScript files that didn’t have corresponding
sources I tracked down the sources and created our own minified files
with the minify-build-system (see #27924).

--
Ricardo

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